The present invention relates to a device for supporting the medial arch of the foot. More particularly, the present invention relates to a device for adjusting the perimeter of the transverse girth of a shoe in the midfoot region under varying conditions of loading. The effective volume of the midfoot area of the shoe is dynamically varied by the present invention to prevent the foot from everting and to provide different levels of support during the gait cycle.
In an attempt to understand the foot as a system, the various parameters which affect the function of the foot have been studied, particularly with regard to a weight bearing foot. The practical need for such knowledge lies in the fact that a true structural model of the foot is capable of providing a prediction of gait and the effects of a shoe on gait. By knowing, in advance, how a shoe would affect the performance of an athlete, for example, optimum shoes could be designed without the usual "cut and try" method of standard shoe development.
The traditional model of the foot provides for a one column, two-axis model which maintains that the foot under load is a rigid structure with a talocrural (ankle) axis and an apparent subtalar axis. The front of the foot is relatively rigid, but with only a multitude of small bone movements about the midtarses axes. The average direction of the effective axis under the ankle, called the subtalar axis, is said to be 42 degrees vertical and 16 degrees horizontal to the midline of the body, as measured by Inman, V. T., The Joints of the Ankle, The Williams & Wilkins Co., Baltimore, 1976. However, this theory does not hold up with regard to a weight bearing or loaded foot since, if the force due to body weight were to act on the single traditional subtalar axis, the foot would collapse mechanically.
It has now been determined that the foot is comprised of two columns and three axes. The lower, lateral column is basically a rigid base comprised of the Calcaneus, Cuboid, and the fourth and fifth metatarsals. The remainder of the foot, which is comprised of the navicular, the first, second and third cuneiforms and the first, second and third metatarsals, emanates from the talus at the talonavicular interface swinging in combination with the lower column inversion/eversion actions in what may be called the `subtalar joint axis`. But this articulation of what is called the upper foot column is only secondary to the true foot mechanism. The primary mechanical loading interface is on the lower, lateral column at the rear of the talus onto the calcaneus, the posterior talocalcaneal facet.
It has also been determined that the foot operates differently under load than when it is passively manipulated such as a doctor would do in the office. This distinction helps to explain previous misconceptions as to how the foot works under load.
This new understanding has yielded a new structural model of the foot which has two separate columns, wrapped together with fascia, and three nearly orthogonal axes. The three axes are: (1) the talocrural (ankle) axis; (2) the talocalcaneal axis (formed at the facet between the talus and the calcaneus); and (3) the talonavicular axis (formed at the facet between the talus and the navicular bones).
Generally, shoes are laced or strapped with no load on the foot until the wearer subjectively feels sufficient tension. Upon loading of the foot such as while walking or running, the girth stretches and the knots tighten. This loosens the girthing tension by an amount which cannot be predicted in advance.
By the present invention there is provided an improved girthing support which allows the creation of a supportive girthing when the foot everts and loads the region of the medial arch. The girthing support of the invention then relaxes when the foot is standing on the lateral border or is unloaded. The present invention allows the degree to which the girth is tightened and loosened while wearing the shoe to be accurately predicted in advance.
In accordance with the present invention, when the foot is in an unloaded condition, the dynamic girth construction of the present invention has no effect on the original girthing tension. Then as the foot is loaded and everted, the girthing fibers are forced into an undulating surface. This reduces the effective length of the fibers and begins to tighten the girth. When the everted foot becomes loaded completely, the girthing fibers are mated with the undulating surface in close contiguous contact, thus pulling in the girth fibers and providing maximum tension on the midfoot region of the foot.
Accordingly, it is a primary object of the present invention to provide a girth structure which will create girthing tension dynamically to prevent excessive eversion when the foot loads the shoe.
An additional object of the invention is to provide a girthing structure which will advantageously interact with the dynamics of the foot while wearing the shoe during walking, running and other activities.
A further object of the invention is to provide a girthing action which will increase the circulation of blood in the feet and assist the heart in the movement of blood in the lower extremities.